597,5 
cop.  2 


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ILLINOIS  STATE  LABORATORY 


OF 


NATURAL  HISTORY 


URBANA,  ILLINOIS,  U.  S.  A. 


STEPHEN  A.  FORBES,  PH.  D.,  LL.D. 

DIRECTOR 


FRESH  WATER  FISHES  AND  THEIR  ECOLOGY 


BY 
STEPHEN  A.  FORBES 


1914 


FRESH  WATER  FISHES  AND  THEIR  ECOLOGY* 
BY  STEPHEN  A.  FORBES 

When  we  watch  a  summer  thunder  storm,  which  covers  the  earth 
with  a  sudden  flood  and  makes  rivulets  by  the  road-side,  each  carrying 
down  to  the  smaller  streams  its  load  of  leaves  and  other  organic  debris, 
together  with  the  lighter  parts  of  the  soil,  and  when  we  see  these  silt- 
laden  streams  unite  in  rivers  turbid  with  the  rich  spoil  of  the  land,  we 
are  inclined  to  lament  the  enormous  and  oft-repeated  waste,  seeing  no 
way  in  which  it  can  be  recovered  in  any  considerable  measure  to  the  use 
of  man ;  but  if  we  follow  it  to  the  lake  bottom  and  the  river  bed  we  shall 
see  much  of  it  arrested  there,  to  become  an  aquatic  soil,  partly  muddy 
water  and  partly  wet  mud,  more  fertile  even  than  the  richest  fields,  and 
sustaining  a  new  population  of  plants  and  animals,  of  many  grades  and 
classes,  one  climbing  upward,  as  we  may  say,  upon  the  shoulders  of 
another,  to  reach  a  level  which  makes  them  accessible  again  to  our  use. 

Since  the  waters  which  wash  the  surface  of  the  earth  fall  virtually 
lifeless  and  sterile  from  the  sky,  whatever  population  they  eventually 
contain  must  evidently  be  supplied  from  the  contributions  made  to  them 
by  the  earth,  including,  of  course,  the  organic  and  inorganic  substances 
dissolved  out  of  the  earth  by  surface  wash  and  underground  nitration. 
The  aquatic  population  of  a  lake  or  stream  is  thus  sustained  by  the  wastes 
of  the  land — materials  which  would  otherwise  be  carried  down  prac- 
tically unaltered  to  the  sea ;  and  our  rivers  and  lakes  may  be  looked  upon 
as  a  huge  apparatus  for  the  arrest,  appropriation,  digestion,  and  assimila- 
tion of  certain  raw  materials  about  to  pass  from  our  control,  valueless 
and  sometimes  deleterious  as  they  leave  us,  but  capable  of  being  worked 
over,  renovated,  and  returned  to  us  in  new  and  valuable  forms,  mainly 
as  fishes  available  for  food. 

The  raw  materials  thus  contributed  by  the  land  vary  according  to 
their  origin.  In  uncivilized  nature  they  were  mainly  the  washings  and 
sweepings  of  the  primitive  prairie  and  forest,  rich  in  carbon  but  with  a 
minimum  amount  of  nitrogen.  With  the  occupation  of  the  country,  the 
cultivation  of  its  lands,  and  the  building  of  towns  and  cities,  the  animal 
wastes  are  increased,  with  their  larger  increment  of  nitrogen,  and  larger 
quantities  of  the  soil  itself  are  swept  into  the  streams — all  alike  available 

*  Read  at  the  University  of  Chicago,  August  20,  1913. 


in  the  end  as  containing  food  for  aquatic  plants  and  animals,  and  so 
finally  for  the  support  of  man.  Even  the  sewage  of  great  cities  is  to  be 
classed  with  the  rest  as  an  available  resource,  capable  of  being  arrested, 
redeemed,  and  returned  to  us  in  acceptable  form,  provided  that  certain 
conditions  are  observed  necessary  to  the  protection  of  these  organs  of 
digestion  against  chemical  poisoning  and  against  a  mechanical  overload- 
ing with  more  food  stuffs  than  they  can  continuously  assimilate.  From 
this  point  of  view  we  may  say  that  as  the  land  loses  fertility,  the  waters 
should  gain;  and  if  they  do  not,  it  is  because  of  faulty  management. 

We  may  be  helped  to  an  analysis  and  understanding  of  the  con- 
struction and  operation  of  this  aquatic  apparatus  of  appropriation  and 
assimilation  if  we  so  arrange  the  principal  organisms  of  our  Illinois 
waters  in  the  form  of  a  table  of  feeders  and  their  food,  of  eaters  and 
things  eaten,  that  we  may  see  at  a  glance  for  each  group  of  fresh  water 
animals  both  what  it  feeds  upon  and  what  feeds  upon  it  in  turn.  (Fig.  1.) 

This  table,  T  need  hardly  say,  might  be  indefinitely  complicated  and 
enlarged;  but  I  am  intending  it  only  to  show  the  main  features  of  the 
relationship.  If  I  had  made  it  to  include  details  and  exceptions  and 
organisms  of  secondary  importance,  or  even  internal  parasites,  it  would 
have  been  too  complex  for  our  present  purpose. 

Notice  especially  the  evident  predominance  of  fishes  in  this  scheme 
of  vital  relationship,  shown  by  the  fact  that  they  feed  upon  everything 
in  the  bill  of  fare  from  terrestrial  wastes  to  frogs,  while  they  are,  on 
the  other  hand,  their  own  worst  enemies,  more  fishes  falling  a  prey  to 
other  fishes  than  to  all  other  aquatic  enemies  combined.  Further,  if  we 
take  account  not  only  of  the  food  of  fishes,  but  also  of  the  food  of  their 
food,  we  shall-  see  that  it  covers  every  item  on  our  table  excepting  a  few 
at  the  lower  right-hand  corner  relating  to  turtles,  serpents,  birds,  and 
mammals,  including,  of  course,  man ;  and  that  even  these  exceptional 
groups  themselves  all  feed  on  fishes.  It  is  thus  graphically  evident  to  us 
that  to  understand  the  ecology  of  fishes  completely  we  must  study  also 
the  ecology  of  every  class  of  living  things  in  the  midst  of  which  they 
live.  We  must  even  go  outside  the  aquatic  environment  and  analyze  the 
relations  of  fishes  to  their  terrestrial  enemies,  and  to  many  terrestrial 
sources  of  their  food.  To  handle  anything  so  complex,  we  must  have 
the  aid  of  such  groupings  and  classifications  as  our  materials  will  per- 


mil;  and  for  even  so  brief  a  discussion  of  the  topic  as  is  possible  for 
us  today,  a  rough  classification  will  be  useful. 

Ecological  classification  of  animals  may  take  either  one  of  two  princi- 
pal directions,  or  indeed  both  of  them  in  turn.  Ecology  being  the  rela- 
tions of  interaction  between  organisms  and  their  environment,  if  we  take 
it  up  from  the  side  of  the  organism  we  shall  naturally  prefer  a  classifica- 
tion based  on  differences  of  animal  reactions  to  the  same  environmental 
conditions — differences  of  behavior,  that  is — and  our  classification  will 
be  a  habit  classification;  if  it  is  the  environment,  on  the  other  hand, 
which  interests  us  primarily,  we  shall  assemble  our  animals  in  groups 
according  to  their  environmental  preferences,  and  our  classification  will 
be  a  habitat  classification.  The  habit  classification  is  fundamentally 
physiological  and  the  habitat  classification  is  primarily  physical  or  spatial, 
and  the  two  cut  across  each  other,  often  at  right  angles,  each  habitat  con- 
taining associated  animals  of  various  habit,  and  each  habit  group  being 
distributed,  as  a  rule,  over  various  habitats.  A  mill  pond,  for  instance, 
is  a  very  definite  habitat,  but  it  may  contain  fishes  of  every  sort  of  habit ; 
strictly  piscivorous  fishes  form  a  very  definite  habit  group,  one  or  more 
of  which  may  be  found  in  almost  any  kind  of  aquatic  habitat. 

I  do  not  myself  favor  the  attempt  to  reduce  the  facts  and  materials 
of  animal  ecology  to  one  hard  and  fast,  all-including  classification,  such 
as  biologists  attempt  to  establish,  in  the  face  of  almost  infinite  difficul- 
ties, for  descriptive  botany  and  zoology ;  but  I  believe  that  our  ecological 
classifications  should  be  as  various  as  the  objects  we  have  in  view,  being 
made  sometimes  on  one  basis  and  sometimes  on  another,  as  best  serves 
our  purpose  at  the  time.  It  will  serve  my  present  purpose  to  classify 
fishes  first  in  general  terms  according  to  the  principal  elements  of  their 
food,  thus  forming  habit  groups,  without  present  reference  to  their 
habitats. 

I  must  first  acknowledge,  however,  that  fishes  can  not  be  completely 
and  clearly  divided  into  mutually  exclusive  groups  upon  this  basis,  for 
their  choices  of  food  and  their  capacities  for  its  appropriation  are  not 
sufficiently  fixed  and  definite  in  the  different  species  to  make  this  prac- 
ticable. I  can  best  describe  the  actual  situation  by  saying  that  fishes 
have  a  common  body  of  food  resources  of  miscellaneous  character  upon 
which  many  of  them  draw  almost  indiscriminately  according  to  the  cir- 
cumstances at  the  time,  but  that  from  this  common  mass  of  resources, 
habits,  and  capacities  there  is  a  tendency  to  specialize  in  various  direc- 
tions, which  tendency  goes  to  its  limit  in  some  species,  halts  at  various 


4 

intermediate  stages  in  others,  and  in  still  others  is  hardly  discernible  at 
all.  Moreover,  the  food  choices  of  scarcely  any  fishes  are  so  definite  and 
unchangeable  as  to  be  unmixed  and  identical  under  all  conditions,  in  all 
parts  of  their  habitat,  and  at  all  times  of  the  year.  We  shall  find,  indeed, 
the  same  state  of  affairs  when  we  come  to  deal  with  a  habitat  classifica- 
tion; and  if  we  look  the  whole  field  of  ecology  over  we  shall  see  it  rather 
characteristic  of  an  ecological  classification  generally,  especially  on  the 
animal  side.  We  can  nevertheless  profit  greatly  by  such  groupings  of 
our  heterogeneous  data  as  are  still  possible,  if  we  admit  the  limitations 
of  the  scheme  and  understand  their  significance. 

One  of  the  most  peculiar  of  our  food  habitat  groups  contains  the 
gizzard-shad  and  the  stone-roller  as  its  most  notable  representatives,  to- 
gether with  a  few  minnows  less  strictly  limited  to  it,  all  of  them  char- 
acterized by  unusually  long,  convoluted  intestines,  the  gizzard-shad  hav- 
ing also  the  digestive  surface  still  further  increased  by  the  development 
of  a  very  large  number  of  finger-like  caeca  on  its  anterior  section.  These 
fishes  all  discard  intermediary  agents,  and  help  themselves  to  the  raw  ma- 
terials of  their  food  in  the  form  of  the  mere  mud  and  slime  of  the  bot- 
tom, which  contains,  of  course,  a  considerable  quantity  of  organic  debris, 
mostly  of  vegetable  origin.  They  form  the  group  of  the  mud-eaters. 
(Figs.  2,  3,  and  4.) 

The  gizzard-shad,  although  but  little  eaten,  is  one  of  our  most  val- 
uable fishes,  since  it  is  enormously  abundant  in  our  large  waters;  both 
rivers  and  lakes,  competes  with  no  other  species  for  food,  and  is  itself 
the  principal  food  of  our  game  or  predaceous  fishes — the  most  highly 
valued  products  of  our  fisheries.  It  affords,  also,  a  remarkable  instance 
of  a  transformation  or  development  in  the  food  habits  and  resources  of 
fishes,  coincident  with  increase  in  size.  From  the  time  this  fish  hatches 
from  the  egg  until  it  comes  to  an  inch  or  so  in  length  it  is  as  slender  as  a 
minnow,  with  the  alimentary  canal  a  simple  straight  tube.  Still  more 
remarkable,  although  the  mouth  of  the  adult  is  perfectly  toothless,  the 
young  have,  at  this  stage,  a  row  of  conical,  pointed  teeth  upon  the  upper 
jaw.  (Fig.  5.)  Teeth  would  evidently  be  useless  to  it  in  sucking  up 
mud  or  straining  out  plankton  from  the  water;  but  to  the  larva— if  such 
it  may  be  called — they  must  be  very  useful,  for  instead  of  being  a  mud- 
eater  the  fish  is  predaceous  in  this  stage,  its  prey  being  the  minute  animals 
of  the  plankton,  especially  the  Entomostraca,  which  it  pursues  and  cap- 
tures one  by  one  as  a  pike  might  capture  minnows.  With  its  growth  and 
transformation  it  changes  its  habits  slowly,  its  food  becoming  more  mixed 


with  mud;  but  it  develops  effective  gill-rakers  also,  and  we  sometimes 
find  the  stomach  of  the  adult  stuffed  with  a  fairly  clean  plankton. 

The  stone-roller,  although  a  mud-eater  and  especially  equipped  for 
that  function  by  its  long  intestine,  wound  in  a  close  coil  around  the  air- 
bladder,  nevertheless  avoids  muddy  waters  as  a  rule,  preferring  quick 
currents  over  rocky  streams,  from  the  stones  of  which  it  nibbles  and  sucks 
the  sediment  and  slime. 

The  most  remarkable  in  many  ways  of  our  American  fresh-water 
fishes  is  the  Polyodon  or  paddle-fish  (Fig.  6),  and  in  nothing  is  it  more 
peculiar  than  in  the  fact  that,  although  it  is  one  of  our  largest  fishes, 
reaching  a  maximum  length  of  six  feet  and  a  weight  of  a  hundred  and 
sixty  pounds,  it  is  essentially  a  plankton-eater,  feeding  largely,  and  some- 
times almost  wholly,  on  the  smallest  aquatic  animals  and  plants,  for  the 
appropriation  of  which  it  has,  in  its  gill- rakers,  a  straining  apparatus 
scarcely  less  effective  than  that  of  the  whalebone  whale.  To  strain  out 
the  plankton,  it  holds  its  enormous  but  weak-jawed  mouth  wide  open 
as  it  swims  about,  permitting  the  water  to  flow  through  its  very  wide 
gill  slits,  getting  thus  not  only  the  smallest  animals  and  plants,  but  many 
insect  larvae  also  of  kinds  abundant  on  the  open  bottom  in  comparatively 
shallow  water.  It  is,  indeed,  a  living,  fine-meshed,  water-net.  This  fish  is 
our  only  proper  member  of  the  special  class  of  plankton-eaters,  although 
plankton  is  taken  in  quantities  at  times,  especially  in  spring,  by  a  con- 
siderable number  of  other  fishes  of  various  sizes,  all  with  long  and  fine 
gill-rakers — structures  which  have,  in  fact,  no  other  use  than  to  strain 
from  the  water  food  particles  too  small  to  be  taken  in  any  other  way. 
The  crappies  and  certain  other  sunfishes  will  often  so  gorge  themselves 
with  plankton  by  this  means  that  the  bulging  of  their  stuffed  stomachs 
can  be  seen  from  the  outside.  Lake  herring  and  white-fish  are  other 
examples  of  this  class,  not  dependent,  however,  upon  plankton  as  their 
most  important  food. 

While  adult  plankton-eaters  are  thus  relatively  few,  it  is  an  interest- 
ing and  peculiarly  important  fact  that  plankton  is  almost  the  sole  infant 
food  of  nearly  all  our  fresh-water  fishes,  of  whatever  kind  or  adult  food 
habit.  The  hatching  season  of  most  of  our  fresh-water  species  is,  in  fact, 
the  prime  season  of  the  year  for  plankton  production  in  the  shallows  and 
back-waters  where  most  fishes  spawn ;  and  the  minute  mouths  and  gill 
slits  of  the  very  young  are  perfectly  fitted,  without  special  adaptation, 
for  the  capture  of  this  microscopic  prey. 


6 

I  have  already  given  you,  in  the  gizzard-shad,  one  example  of  a 
young  fish  especially  armed  with  teeth  for  this  sort  of  hunting,  and  I 
may  mention,  in  passing,  another  case  of  the  kind  which  is  even  more 
remarkable.  The  common  whitefish  of  the  Great  Lakes  is,  as  you  all 
know,  quite  toothless,  and,  as  an  adult,  is  what  the  Germans  call  a  Klein- 
tierfresser — a  convenient  word  for  which  we  can  hardly  substitute  its 
literal  translation — a  small-animal-devourer.  The  recently  hatched  white- 
fish,  however,  is  a  pure  plankton-eater,  and  it  must  snatch  its  minute 
prey,  one  at  a  time,  from  the  sparsely  inhabited  waters  of  the  open  lake. 
It  is  very  important  to  it,  consequently,  that  it  should  not  miss  its  catch 
or  lose  its  hold,  and  we  find  it  specially  equipped  against  this  accident 
with  four  acute,  curved,  raptatorial  teeth  on  its  lower  jaw  (Fig.  7),  as 
effective  against  a  Cyclops  or  a  Diaptomus  as  the  fangs  of  a  tiger  against 
an  antelope. 

Such  transformations  in  food  habit  with  increasing  size  are,  indeed, 
the  rule  among  fresh-water  fishes.  Starting  together  as  plankton-eaters, 
they  presently  diverge  in  habit,  reaching  their  adult  food  stage  through 
two  or  three  degrees  of  change.  The  sheepshead  (Fig.  8),  for  example, 
begins,  like  the  rest,  with  plankton,  becomes  insectivorous  when  it  is  a 
few  inches  long,  living  almost  wholly  on  the  insect  larvae  of  the  bottom, 
and  as  it  reaches  adult  size  its  habits  change  again  to  those  of  a  mollusk- 
eater,  in  adaptation  to  which  it  develops  in  its  throat  a  powerful  crush- 
ing apparatus,  with  pharyngeal  jaws  capable  of  smashing  the  thickest 
shells  of  our  water  snails,  and  even  those  of  clams  or  mussels  of  con- 
siderable size. 

Other  mollusk-eaters  are  the  Great  Lakes  sturgeon  (Fig.  9)  and 
certain  species  of  the  catfish  (Fig.  10),  sucker  (Fig.  11),  and  sunfish 
families  (Fig.  12),  several  of  them  especially  equipped  for  crushing 
shells — the  suckers  and  sunfish  by  stout,  blunt  teeth  set  in  their  strong 
pharyngeal  jaws  (Fig.  13),  and  the  catfish  by  pads  of  sharp  conical 
teeth  in  their  premaxillaries  (Fig.  14)  and  mandibles.  By  the  use  of 
these  they  seem  able  to  crack  a  snail  as  a  boy  cracks  a  hazelnut,  reject- 
ing the  broken  shells  to  swallow  the  juicy  meats.  Among  the  suckers 
there  is  a  curious  inverse  correlation  in  the  development  of  certain  of  their 
feeding  structures,  gill-rakers  and  pharyngeal  jaws  growing,  one  may 
say,  each  at  the  expense  of  the  other.  That  is,  where  gill-rakers  are 
long  and  numerous,  the  pharyngeal  jaws  are  weak  and  their  teeth  are 
numerous  and  small,  and  the  species  feeds  largely  on  Entomostraca ; 
while  if  the  pharyngeal  jaws  are  thick  and  strong,  with  strong  crush- 


ing  teeth,  the  gill-rakers  are  short  and  thick,  and  relatively  ineffective 
as  a  straining  apparatus.  This  is  so  generally  true  that  one  may  even 
tell  whether  or  not  a  sucker  is  a  mollusk-eater  by  looking  at  its  gill- 
rakers,  although  these  have  nothing  to  do  directly  with  the  collection 
or  mastication  of  molluscan  food. 

Another  terminus  to  the  series  of  changes  in  food  choice  through 
which  most  fishes  pass  is  in  the  piscivorous  habit,  characteristic  of  what 
we  commonly  call  our  game  fishes;  especially  the  pike,  the  pike-perch, 
and  the  Great  Lakes  trout;  but  the  largest  number  of  our  fresh-water 
species  linger  in  the  intermediate,  insectivorous  stage.  Indeed,  taking 
our  adult  fresh-water  fishes  as  they  come,  we  find  that  insects  are  by 
far  the  most  important  general  element  of  the  food  of  the  class,  eaten 
more  or  less  by  nearly  every  kind  of  fish  and  the  main  dependence  of  a 
great  many  species  which,  by  mere  increase  in  size  and  the  consequent 
coarser  structure  of  their  gill  apparatus,  have  lost  the  original  capacity 
of  the  young  to  strain  out  the  plankton,  without  attaining  to  a  size  and 
strength  sufficient  for  the  capture  of  a  prey  larger  and  stronger  than 
aquatic  insect  larvae. 

Some  kinds  of  insects  occur  in  such  abundance  in  situations  diffi- 
cult of  access,  that  certain  groups  of  fishes  have  become  especially  adapted 
to  their  search  and  capture  there.  Darters,  for  example  (Fig.  15), 
live  mainly  on  insect  larvae  which  hide  under  stones  in  swift  water,  and 
they  are  enabled  to  get  at  this  food  by  virtue  of  their  large  pectoral  and 
anal  fins,  by  which  they  can  support  themselves  on  the  bottom  in  a  swift 
current  or  make  their  way  among  the  ripples  of  a  rocky  stream,  and  by 
their  small  heads  and  pointed  noses  which  enable  them  to  pry  about 
under  stones  where  worm-like  Chironomus  larvae  and  larvae  of  small  May 
flies  abound.  A  little  cyprinoid  fish — the  sucker-mouthed  minnow  (Fig. 
16) — is  very  similarly  equipped  and  to  a  like  advantage.  Access  to 
the  same  kind  of  food  under  the  heavier  stones  of  larger  streams  is  given 
to  a  sucker  known  as  the  hammerhead  (Fig.  17).  It  has  a  similar  develop- 
ment of  the  paired  and  anal  fins,  and  a  large  square  head  with  which  it 
can  push  and  roll  about  the  stones  under  which  day-fly  and  stone-fly 
larvae  may  be  found  in  great  abundance ;  and  these  are  its  principal  food. 

Besides  the  six  food  classes  which  I  have  already  mentioned,  namely, 
the  mud-eaters,  the  plant-eaters,  the  plankton-eaters,  the  mollusk-eaters, 
the  insect-eaters,  and  the  fish-eaters,  we  may  doubtfully  distinguish  two* 
more — the  garbage-eaters  and  the  omnivora.  There  is,  indeed,  but  one 
of  our  fresh-water  fishes — the  common  eel — which  seems  to  live  by  pref- 


erence  on  dead  food  or  decayed  flesh;  but  the  class  of  garbage-eaters 
may  be  made  to  include  three  or  four  of  the  catfishes  also,  which  resort 
to  such  food  willingly  when  it  is  convenient  to  them.  Certainly  fishes 
in  whose  stomachs  we  have  found,  from  time  to  time,  distillery  slops, 
ham  bones,  dead  rats,  dead  cats,  and  heads  and  entrails  of  fish  thrown 
out  from  fish  boats,  need  not  complain  if  they  are  provisionally  assigned 
to  the  humble  class  of  scavengers. 

These  same  catfishes  might  perhaps  be  better  classed  as  omnivorous, 
for  they  eat,  in  fact,  very  nearly  every  kind  of  food  which  the  water 
contains,  including  insects,  mollusks,  fishes,  crawfishes,  and  sometimes 
unusual  quantities  of  algae  and  other  aquatic  vegetation.  In  this  omnivo- 
rous class  we  may  also  place  the  common  European  carp,  except  that  this 
fish  does  not  eat  carrion. 

If,  now,  we  review  the  generalities  and  the  peculiarities  of  food 
and  feeding  habits  which  I  have  imperfectly  sketched,  seeking  to  under- 
stand their  differentiation  and  succession,  we  may  best  interpret  the 
facts  by  attempting  to  realize  the  food  resources  of  an  average,  typical, 
undifferentiated  fish,  which  should  reach  adult  condition  without  acquir- 
ing any  special  adaptations  of  structure  or  of  preference  in  respect  to  the 
choice,  appropriation,  and  assimilation  of  its  food.  Such  an  undiffer- 
entiated fish  would  have  a  subcylindrical  body  with  only  the  ordinary 
equipment  for  locomotion;  it  would  be  toothless  both  as  to  its  jaws  and 
its  pharyngeal  bones ;  its  mouth  would  be  neither  suctorial  nor  especially 
protractile;  and  its  gill-arches  would  be  without  specialized  gill-rakers. 
In  other  words,  it  would  be  a  simple  product  of  growth,  without  progress 
or  differentiation,  from  the  state  of  the  recently  hatched  fry.  Such  a 
fish  would  necessarily  begin,  as  all  our  fishes  now  do,  with  a  mixed 
plankton  for  its  earliest  food,  taking  the  smaller  organisms  first  and  the 
larger  ones  later.  As  it  gradually  becomes  too  large  for  the  pursuit  of  so 
minute  a  prey,  and  its  gill  structures  too  coarse  to  serve  longer  as  a  plank- 
ton strainer,  it  would  draw  next  upon  the  insects,  and  mainly  on  the 
insect  larvae  of  the  bottom  and  the  shores — creatures  especially  avail- 
able to  it  because  their  soft  and  poorly  protected  bodies  make  them  fit 
for  digestion  without  mastication  or  other  special  preparation ;  and  with 
these  it  might  mingle  also  amphipod  crustaceans,  and  the  smaller  thin- 
shelled  mollusks,  especially  those  which  could  be  picked  from  an  aquatic 
vegetation.  Next  would  come  such  young  fishes  as  it  could  seize  and 
swallow  without  a  special  armature  of  jaws  and  throat;  and  at  this  stage 
of  growth  and  progress  it  would  apparently  stop.  To  go  farther  as  a 


0 

predaceous  fish  it  would  need  the  swimming  capacity  and  the  raptatorial 
teeth  of  a  pike-perch  or  a  pike;  to  get  effective  access  to  the  abundant 
stores  of  molluscan  life,  gastropod  and  bivalve,  in  our  streams  and 
lakes,  it  would  need  either  a  suctorial  mouth  or  strong  pharyngeal  jaws 
with  crushing  teeth,  or  both  of  these,  and  thus  it  might  become  the 
equivalent  of  a  sheepshead,  a  sunfish,  or  a  sucker,  as  other  conditions 
should  determine.  To  continue  as  a  plankton-eater,  it  would  need  the 
numerous,  long,  and  slender  gill-rakers  of  a  paddle-fish  or  a  lake  herring, 
and  with  these,  especially  if  it  had  a  suctorial  mouth  and  a  very  long  in- 
testine, it  might  be  able  to  sift  and  strain  from  the  silt  of  the  bottom 
the  finer  organic  particles  derived  from  the  debris  of  aquatic  vegetation 
and  from  the  wash  of  the  land.  Other  specialties  of  differentiation  in 
structure  or  in  habit  might  open  to  it  less  usual  food  resources,  as  with 
the  darters  and  the  top-minnows ;  and  a  mere  deviation  or  degradation 
of  taste  might  add  the  carrion  of  the  stream  to  its  menu. 

Evolution  of  food  habits  must  thus  have  taken  the  course  of  struc- 
tural evolution — an  advantageous  specialization  in  various  degrees  and 
in  various  directions  from  a  generalized,  undifferentiated  original.  Some- 
times added  specialties  of  advantageous  equipment  have  brought  in  their 
train  limitations  or  prohibitions  in  other  directions,  which  have  shut  a 
species  out  from  certain-  food  resources  in  making  others  more  available. 
The  same  set  of  gill-arches,  for  example,  can  not  serve  at  once  as  a 
plankton-net  and  a  shell-crusher;  but,  generally  speaking,  the  structural 
differentiations  mentioned  have  enlarged  the  resources  of  the  fish  in  some 
directions  without  reducing  them  in  others.  Even  so  definitely  predaceous 
a  fish  as  the  Great  Lakes  trout,  which  lives  habitually  on  the  abundant 
herring  of  the  lakes,  has  been  known  to  devour  salt  pork,  ham  bones, 
chicken  bones,  raw  potatoes,  corn  cobs,  rags,  spoons,  tin  cans,  silver 
dollars,  and  in  single  instances,  a  watch  and  chain,  an  open  jack  knife 
seven  inches  long,  and  a  two-foot  piece  of  tarred  rope. 

From  this  it  would  appear  that  these  structural  differentiations 
have  not  necessarily  followed  upon  differentiations  of  preference,  fitting 
the  fish  to  get  more  easily  and  abundantly  the  kind  of  food  which  it  had 
already  come  to  prefer;  they  seem  to  have  arisen  independently  of  any 
peculiarities  of  choice,  and  may,  indeed,  have  forced  the  species,  in  a 
sense,  into  directions  which  it  would  not  otherwise  have  been  inclined 
to  follow. 


10 

If  we  turn  now  from  these  examples  of  habit  groups  to  a  classi- 
fication by  habitats — from  physiological  to  spatial  ecology — from  a  dis- 
cussion of  the  food  of  fishes  to  the  subject  of  their  local  distribution  and 
their  assemblage  in  what  are  called  animal  associations,  we  shall  find  a 
similar  state  of  affairs  to  that  just  noticed.  Some  of  our  fresh-water  fishes 
are  so  widely  and  thoroughly  distributed  over  a  large  variety  of  situa- 
tions that  they  may  be  likened  to  the  omnivorous  class  in  the  classifica- 
tion by  habits,  while  others  are  as  narrowly  limited  in  habitat  as  is  the 
carnivorous  pike  in  respect  to  its  food.  For  my  detailed  data  of  local 
distribution  I  shall  have  to  draw  almost  wholly  on  our  Illinois  observa- 
tions, for  the  reason  that  we  have  made  in  Illinois  much  larger  and  more 
intensive  collections  of  our  native  fishes  than  have  been  made  in  any 
other  state — larger,  indeed,  as  I  believe,  than  in  any  other  area  of  like 
size  anywhere  in  the  world. 

The  blunt-nosed  minnow  (Fig.  18)  is  an  example  of  what  we  may 
call  omnilocal  distribution,  the  map  of  its  local  occurrences  in  Illinois 
(Map  XXVII),  being  a  fair  abstract  of  the  map  of  localities  for  all  our 
Illinois  fish  collections.  It  is  relatively  rare  only  in  our  larger  rivers, 
the  frequency  of  its  occurrence  there  being,  by  our  data,  as  5  to  34  for 
the  smaller  rivers,  and  to  43  for  creeks.  That  is  to  say,  if  we  were  to 
take  equal  numbers  of  fish  collections  from  each  of  these  classes  of 
waters  throughout  the  state  until  we  found  this  species  five  times  in 
larger  rivers,  we  might  expect  to  find  it  about  thirty- four  times  in  small 
rivers,  and  about  forty-three  times  in  creeks.  It  is  not  limited  in  its  range 
or  habitat  by  its  choice  of  food,  for  it  feeds  mainly  on  mud,  and  that  it 
could  easily  find  almost  anywhere  in  Illinois.  It  prefers  streams  with 
a  rocky  bottom,  it  is  true,  its  occurrence  in  such  waters  having  a  fre- 
quency of  46  as  compared  with  27  in  other  places  ;  and  the  kinds  of  vegeta- 
tion mixed  with  the  mud  of  its  intestinal  contents  give  us  reason  to  think 
that  it  nibbles  and  sucks  the  slime  from  stones  and  other  submerged 
objects. 

Contrast  with  this,  now,  the  distribution  map  of  the  spot-tailed  min- 
now (Map  XXXVIII),  and  another  minnow  species,  Notropis  heterodon 
(Map  XXXIV),  not  well  enough  known  to  have  received  an  English 
name.  The  spot-tailed  minnow,  very  common  in  lakes  and  ponds  and 
especially  in  the  Great  Lakes,  occurs  elsewhere  mainly  in  the  larger 
rivers,  its  average  frequency  in  our  collections  in  these  two  situations 
being  as  33  to  3.5  in  the  smaller  streams.  That  is,  we  have  found  it  nearly 
ten  times  as  common  in  the  larger  waters  as  in  the  smaller  ones ;  and  its 


11 

food,  consisting  mainly  of  insects,  crustaceans,  algae,  and  fragments  of 
aquatic  plants,  is  as  different  from  that  of  the  preceding  species  as  is  its 
distribution.  The  little  heterodon,  on  the  other  hand — about  two  inches 
long — is  essentially  a  lake  and  pond  species,  and  its  abundance,  not  only 
in  the  lake  region  of  northeastern  Illinois  but  also  along  the  larger  rivers, 
is  explained  by  the  fact  that  it  is  mainly  in  the  lowlands  of  the  river  bot- 
toms that  lakes  and  ponds  are  to  be  found  in  Illinois.  It  is  in  such  sit- 
uations that  it  finds  the  bottoms  of  mud  and  sand  which  seem  to  attract 
it,  its  frequency  ratio  there  being  71  as  compared  with  22  over  rock  and 
sand,  and  7  over  mud.  The  food  of  this  species  is  consistent  with  this 
preference  of  location,  being  mainly  Entomostraca  and  small  larvae  of 
gnats.  It  is,  indeed,  essentially  a  plankton-eater,  being  of  the  size  to 
make  the  plankton  of  its  favorite  resorts  its  most  convenient  and  abundant 
food. 

I  know  well  that  these  specific  details  are  hardly  fit  for  a  general 
lecture,  but  they  are  the  materials  of  my  generalizations,  and  I  must  ask 
you  to  indulge  me  to  the  extent  of  two  more  examples,  chosen  from  an- 
other family  of  fishes — that  most  interesting  division  of  the  perches 
commonly  known  as  the  darters.  These  are  the  johnny  darter  (Fig. 
19;  Map  XC),  and  another  species,  Cottogaster  shumardi  (Fig.  20;  Map 
LXXXVIII),  which  has  no  English  name.  They  are  particularly  inter- 
esting, because  the  johnny  darter,  although  very  abundant  all  over  the 
state,  seems  to  avoid  the  larger  streams,  having  a  frequency  there  of 
only  3  as  against  53  for  creeks,  while  the  Cottogaster,  although  compara- 
tively rare,  occurs  almost  wholly  in  the  larger  rivers  and  in  the  bottom- 
land lakes  in  their  immediate  neighborhood.  Whether  there  are  differ- 
ences in  food  corresponding  to  their  distribution  we  cannot  tell,  be- 
cause the  food  of  the  rarer  species  has  not  been  studied. 

Many  other  instances  might  be  given  of  the  fact  that  fishes  can  be 
separated  into  groups  according  to  their  habitats  as  well  as  by  differences 
in  their  food,  but  that  the  groups  so  formed  are  of  very  unequal  scope. 
It  is  as  if,  in  classifying  fishes  structurally,  we  should  find  that  there 
were  some  families  which  combined  the  characteristics  of  nearly  all  the 
others ;  that  other  kinds  present  many  such  common  characters,  but  a 
smaller  number;  and  that  only  a  few  had  differentiated  so  far  from  the 
common  mass  as  to  have  fixed  distinguishing  characters  of  their  own. 
An  ecological  classification,  while  quite  possible,  and  indispensable  also, 
ought  not  to  be  framed  in  imitation  of  the  classifications  of  the  taxonomist, 
but  must  have  objects  and  methods  of  its  own. 


12 

You  are  all  familiar,  no  doubt,  with  the  idea  of  animal  associa- 
tions— groups  of  species  habitually  associated  in  the  same  environments ; 
and  these  are  as  recognizable  among  fishes  as  among  the  animals  of  the 
land ;  and  here  again,  substantially  as  in  the  other  cases,  we  find  animals 
which,  taken  by  themselves,  may  be  seen  to  form  associate  groups 
•with  a  large  extension,  covering  many  distinguishable  kinds  of  situations, 
and  others  which  are  rather  narrowly  limited  to  a  single  sort  of  habitat, 
characterized  by  quite  special  conditions.  There  is  a  group  of  creek 
fishes,  for  example,  made  up  of  species  which  may  be  found  almost  any- 
where in  a  creek  and  in  almost  any  kind  of  a  creek,  and  another  group, 
like  certain  of  the  darters  and  the  stone-roller,  which  are  thoroughly 
at  home  only  in  rocky  streams  with  a  relatively  rapid  flow  of  at  least 
fairly  clear  water.  If  we  analyze  the  aquatic  environment  into  all  the 
situations  clearly  distinguishable,  we  shall  find,  perhaps,  a  group  of  cer- 
tain species  distinctive  for  each  situation,  but  for  other  species  our 
analysis  will  be  seen  to  have  gone  too  much  into  detail ;  it  will  distinguish 
differences  of  condition  to  which  they  are  indifferent.  Even  the  distinc- 
tion between  small  river  and  creek,  and  between  river  and  lake,  is  too 
narrow  for  some  fishes,  which  are  found  with  almost  equal  frequency  in 
both.  The  grass  pickerel  (Fig.  21),  which  we  have  taken  in  one  hundred 
and  eleven  Illinois  collections,  is  almost  equally  abundant  in  creeks  and 
in  ponds;  and  the  river-chub  (Fig.  22)  has  been  found  about  equally 
common  in  creeks  and  in  small  rivers,  and  virtually  absent  from  the  larger 
rivers  and  from  lakes.  Nevertheless,  the  distinction  of  animal  associa- 
tions is  very  helpful  to  our  grasp  and  understanding  of  the  system  of 
living  nature,  and  will  become  much  more  so  as  our  knowledge  becomes 
both  more  comprehensive  and  more  precise. 

An  organization  of  the  animal  population  of  a  region  into  associa- 
tions may  be  approached  either  from  the  side  of  the  environment  or  from 
that  of  the  animal  inhabitants;  either  by  an  analysis  of  the  environment 
into  habitats  and  situations,  and  a  critical  survey  of  the  inhabitants  of 
each,  or  by  an  analysis  of  the  animal  population  into  groups  of  most  fre- 
quent associates,  and  a  study  of  the  local  and  ecological  distribution  of 
each  such  group.  By  the  first  method,  spatial  units — so-called  units  of 
environment — are  first  distinguished  and  delimited,  and  the  animals  con- 
tained in  each  such  unit  are  then  identified,  listed,  and  enumerated.  By 
the  second  method,  the  associate  groups  are  first  distinguished,  defined, 
and  analyzed,  and  the  territory  under  examination  is  mapped  in  a  way  to 
mark  the  area  of  distribution  of  each  such  group.  The  first  method  is 


13 

primarily  geographical,  and  the  second  is  essentially  biological.  Both 
methods  are  useful  and  the  product  of  both  is  necessary  to  a  full  knowl- 
edge of  our  complex  subject;  but  of  the  two,  the  biological  method  seems 
to  me  much  the  more  useful  and  significant. 

The  most  general  division  of  the  environment  of  fresh-water  fishes 
into  habitats  distinguishes  large  rivers,  small  rivers,  creeks,  upland  lakes, 
lowland  lakes,  marshes,  and  stagnant  ponds.  Streams  are  still  further 
divisible  into  those  with  rocky  bottoms,  bottoms  of  sand,  and  bottoms  of 
mud;  into  those  with  a  swift  or  with  a  sluggish  current;  and  into  those 
with  clear  or  with  turbid  waters.  Even  parts  of  streams  are  distinguish- 
able into  different  habitats — the  rippled  reaches  of  rocky  streams  differ  ma- 
terially from  the  deep,  still  pools  between.  In  the  larger  rivers,  like  the 
Illinois,  we  may  sometimes  distinguish  between  the  opposite  margins, 
where,  as  at  Havana,  one  has  a  muddy  bank  and  the  other  a  sandy  one. 
In  lakes  there  are  notable  differences  between  the  marginal  shoals  and 
the  deep  interior  waters,  between  sandy  bars  and  mud  flats;  between 
open  waters  and  those  filled  with  weeds ;  and  in  the  weedy  parts,  be- 
tween those  in  which  reeds,  rushes,  and  other  coarse,  rooting  plants  are 
present  and  those  in  which  the  plants  are  mostly  submerged.  These  pre- 
sent their  characteristic  differences  in  the  fishes  which  resort  to  them — 
differences  clearly  discernible,  however,  only  by  the  use  of  quantitative 
methods,  which  give  us  the  relative  numbers  of  each  species  found  in 
each  situation  over  a  sufficient  length  of  time  and  variety  of  external 
conditions  to  make  us  sure  that  we  are  getting  fair  and  stable  averages. 

The  first  thoroughly  practical  work  of  this  kind  that  I  know  of  was 
done  at  Havana,  Illinois,  under  my  direction,  in  1898-99,  by  Wallace 
Craig,  later  a  doctor  of  philosophy  of  the  University  of  Chicago,  but  at 
that  time  a  temporary  assistant  on  the  Illinois  Natural  History  Survey 
and  also  a  graduate  student  in  the  University  of  Illinois.  He  began  in 
August,  1898,  a  detailed  study  of  the  local  distribution  and  the  move- 
ments of  fishes,  with  a  view  to  making  out  preferences  of  situation  or 
choices  of  environment  of  the  various  species  of  fish  under  varying  con- 
ditions and  at  different  times  of  the  year.  Using  identical  apparatus  by 
uniform  methods  at  regular  intervals  in  the  waters  of  the  locality,  it  was 
possible  to  get  totals  and  averages  by  a  comparison  of  which  the  strik- 
ing features  of  the  different  situations  were  made  manifest  when  the 
statistical  tests  were  compared  with  each  other.  By  this  method  it  was 
shown  that  the  Illinois  River  gave  us  different  data  of  frequency  for 
the  different  species  on  the  two  sides  of  the  stream,  one  of  which  was 


14 

muddy  and  the  other  clear;  but  the  most  interesting  conclusion  was  a 
notable  difference  in  degree  of  specialization  in  the  fishes  inhabiting  the 
different  sections  of  a  stream  system.  Those  from  the  larger  river  were, 
as  a  rule,  not  only  the  largest,  but  the  most  primitive,  or  the  least  special- 
ized; those  preferring  the  bottomland  lakes  were,  on  the  whole,  more 
highly  differentiated;  and  those  from  the  creeks  were  smallest  and  the 
most  highly  specialized  of  all. 

It  is  perhaps  what  we  ought  to  expect,  that  the  creek  species  should 
be  more  diverse  and  highly  organized  than  any  other  fishes,  for  they 
must  have  had  longer  experience  of  fresh-water  life.  As  the  continent 
first  began  to  rise  from  the  sea,  all  its  streams  were  necessarily  small, 
and  its  first  fishes  were  consequently  those  adjusted  to  life  in  creeks. 
As  the  process  continued,  as  the  surface  of  the  land  became  more  di- 
versified, as  the  small  stream  systems  of  the  coast  ate  their  way  back, 
with  many  lateral  branches,  and  united  to  form  large  rivers,  and  these 
again  to  make  rivers  of  the  largest  size,  new  habitats  would  be  formed, 
both  in  the  uplands  and  along  the  coast,  and  new  adaptations  of  fishes 
to  them  would  naturally  lead  to  about  the  kind  of  classifiable  diversity 
which  we  actually  find. 

That  ecological  differentiations  and  divisions  among  fresh-water 
fishes  are,  as  a  rule,  of  no  very  compelling  force  is  shown  in  a  remark- 
able manner  by  the  fact  that  the  whole  system  of  such  distinctions  breaks 
down  almost  completely  at  least  once  a  year,  when  a  great  migration 
movement  up-stream  and  into  shallow  water  seizes  all  species  alike,  under 
the  overpowering  impulse  of  the  breeding  instinct.  At  this  time  fishes 
of  the  most  varied  habit  and  habitat  seem  temporarily  to  desert  or  forget 
their  favorite  places  of  resort,  and  throng  together,  indifferent  to  their 
individual  welfare,  in  search  of  places  for  the  deposit  of  their  eggs, 
and,  with  many  species,  for  the  subsequent  care  and  protection  of  their 
young.  Even  under  less  extraordinary  circumstances  I  have  found,  in 
fact,  that  a  large  river  like  the  Illinois  becomes  a  sort  of  metropolis  of 
the  fish  population  of  its  drainage  basin,  in  which  representatives  of  the 
various  groups  or  associations,  separate  and  distinct  in  its  headwaters 
and  smaller  tributaries,  may  be  found  indiscriminately  commingled,  just 
as  in  this  great  city  we  see  people  from  scores  of  smaller  cities  and 
hundreds  of  smaller  towns  and  thousands  of  rural  communities.  Sunfish 
species,  for  example,  which  rarely  occur  in  each  other's  company  in 
collections  from  the  smaller  tributaries,  were  found  together  twice  as 
often  in  collections  from  ihe  larger  streams. 


15 

Such  are  some  of  the  products  of  a  study  of  the  populations  of 
predetermined  habitats.  It  remains  to  be  seen  to  what  extent  these 
habitat  populations  coincide  with  real  ecological  groups  of  most  fre- 
quent associates — to  what  extent  our  habitat  characters  are  the  real  de- 
terminers of  the  actual  associative  distribution  of  our  fishes.  It  may 
be  that  the  effective  sensibilities  of  fishes  are  not  altogether  what  we 
have  supposed  them  to  be,  a  priori;  that  things  we  have  not  thought  of 
in  that  connection  have  much  to  do  with  their  assemblage  in  more  or 
less  definite  and  in  more  or  less  permanent  societies.  This  is  especially 
possible  with  fishes,  because  we  can  see  so  little  of  them  as  a  rule,  and 
because  their  power  of  free  and  rapid  locomotion  enables  them  to  as- 
semble and  to  disperse  so  readily  and  so  rapidly.  To  get  at  the  funda- 
mental facts  we  must  find  a  means  of  learning  what  and  where  definite 
associations  among  fishes  really  exist;  what  is  the  local  center  and  what 
are  the  optimum  conditions  of  each  such  associate  assemblage ;  and  what 
are  its  most  typical  and  constant  components — what  species,  that  is  to 
say,  are  the  most  clearly  and  constantly  characteristic  of  it.  This  means 
that  we  must  study  the  details  of  the  distribution,  and  hence  of  the 
associate  grouping  of  fishes,  with  reference  at  first  to  their  location  only ; 
and  then,  when  our  associations  have  thus  been  determined,  located,  and 
described,. we  must  see  how  they  compare  with  the  habitat  system  ar- 
rived at  by  our  preliminary  analysis  of  the  environment. 

This  sort  of  critical  study  of  the  essential  details  of  ecological  dis- 
tribution has  almost  never  been  made,  at  least  for  animals,  and  even 
the  methods  of  it  are  scarcely  agreed  upon.  Those  which  I  shall  briefly 
describe  to  you  were  devised  for  the  purpose  of  utilizing,  for  ecological 
description  and  inference,  the  product  of  extensive  collections  of  Illi- 
nois fishes,  made  in  the  long  course  of  the  natural  history  survey  of 
the  state.  They  are  based  upon  the  obvious  fact  that  a  biological  asso- 
ciation is  made  up  of  species  which  are  associated  with  one  another  more 
frequently  than  they  are  with  other  species ;  from  which  it  follows  that 
to  find  an  association  one  must  find  a  group  of  such  most  frequent  asso- 
ciates ;  and  to  determine  the  center  of  its  location  and  the  extent  of  its 
range  we  must  find  where  this  associative  frequency — this  frequency  of 
joint  occurrence — of  the  several  species  of  the  group  is  greatest,  and 
how  far  in  each  direction  each  species  of  it  continues  to  be  more  fre- 
quently associated  with  the  other  members  of  the  group  than  with  any 
other  species.  If,  for  example,  we  make  a  hundred  collections  over  a 
given  area  of  complex  ecological  composition,  and  find  that  we  have 


16 

fifty  species  of  fish  represented  in  these  collections,  it  is  easy  for  us  to 
tell,  by  a  simple  examination  of  our  numerical  data,  which  of  these  fifty 
species  have  come  out  in  our  nets  in  each  other's  company  the  most  fre- 
quently, and  in  what  situation  or  habitat  each  of  these  most  frequent 
associates  has  been  found  most  abundant.  In  so  far  as  frequency  of 
habitat  occurrences  and  frequency  of  associate  occurrences  coincide,  we 
have  evidently  a  local  association  distinguished,  together  with  its  char- 
acteristic and  accustomed  habitat. 

I  tested  the  utility  of  these  simple  ideas  during  the  summer  vaca- 
tion of  1905,  by  an  application  of  them  to  our  Illinois  collections  of  the 
so-called  darters — species  of  the  subfamily  Etheostominse — in  a  way  to 
prove,  what,  indeed,  we  already  knew  as  a  matter  of  common  observation, 
that,  taken  as  a  whole,  these  darters  are  an  associate  group,  and  that 
their  characteristic  habitat  is  what  also  we  already  knew  it  to  be — the 
rocky  rapids  of  small  streams.  The  essential  correctness  of  the  method 
was  thus  verified ;  and  I  was  also  able  to  distinguish  six  species  of  darters 
peculiarly  typical  of  the  group,  to  be  regarded  as  especially  characteristic 
of  it  because  they  were  found  more  than  two  and  a  fourth  times  as  fre- 
quently associated  with  each  other  as  they  were  with  the  seven  remain- 
ing species;  and  likewise  because  they  were  about  two  and  a  half  times 
as  frequently  associated  with  each  other  as  were  the  seven  remaining 
species  among  themselves. 

It  was  easy  to  show,  on  the  other  hand,  by  similar  methods,  that 
the  sunfishes  (see  Fig.  23  and  24),  although  as  much  alike  to  a  general 
observation  as  the  darters,  are  not  a  homogeneous  ecological  group,  but 
that  they  are  so  variously  related  to  different  habitats — to  different  fea- 
tures of  their  environment — that  several  of  the  sunfish  species  are  much 
more  frequent  associates  of  fishes  widely  different  from  themselves  than 
they  are  of  each  other;  that  the  various  sunfish  species  often  belong,  in 
fact,  to  different  zoological  associations.  Indeed,  it  was  found  by  re- 
peated use  of  this  method  of  analysis  that  it  was  a  rather  common  thing 
for  closely  related  species  of  fishes — near  neighbors  in  the  taxonomic 
system — to  be  in  some  sense  averse  to  each  other's  company — to  avoid 
each  other,  seemingly,  and  to  find  their  closest  and  most  familiar  asso- 
ciates in  fishes  far  removed  from  them  in  taxonomic  relationship. 

I  have  supposed  this  to  be  an  expression  of  the  disadvantages  of 
close  competition  between  closely  similar  species,  and  of  the  advantage, 
consequently,  of  such  differentiations  in  habit  and  such  separations  in 
ecological  preference  as  would  carry  these  natural  competitors  into  non- 


competing  ecological  groups.  Such  an  apparent  evasion  of  competition 
by  near  relatives  is  well  illustrated  by  our  observations  on  the  top- 
minnows  (Fig.  25) — very  small  fishes  of  the  killifish  family,  of  which 
we  have  three  species  in  Illinois.  Two  of  these  species  are  distributed 
throughout  the  state,  but  the  third  is  southern  in  its  distribution,  lapping 
over  on  the  Illinois  area  of  the  other  two  only  in  the  southern  part  of 
the  state.  Now  it  was  to  me  an  extremely  interesting  fact  that  we  found 
this  southern  species  much  more  frequently  in  company  with  the  two 
more  widely  distributed  top-minnows  than  these  two  were  with  each 
other.  It  was  as  if  those  which  occupy  the  same  area  conjointly  had  found 
themselves  compelled  to  an  ecological  division  of  it,  such  as  would  keep 
them  largely  out  of  each  other's  way,  while  those  of  an  essentially  un- 
like geographical  distribution  had  found  no  such  mutual  avoidance  nec- 
essary. It  seems  quite  possible  that  this  intra-local  separation  of  com- 
petitive groups,  this  effective  isolation  of  forms  inhabiting  the  same  ter- 
ritory, may  be  one  of  the  early  steps — sometimes  the  very  first  step, 
perhaps — in  the  differentiation  and  fixation  of  species.  A  difference  in 
respect  to  choice  of  breeding  grounds  especially,  by  preventing  the  inter- 
breeding of  two  diverging  groups,  would  separate  them  as  effectually 
as  an  impassable  mountain  chain  running  through  the  area  of  their 
original  distribution. 

This  principle  of  an  evasion  of  competition  seems  to  apply  to  asso- 
ciations also,  as  well  as  to  species,  and  to  explain  in  part  the  composi- 
tion of  neighboring  associations.  Similarly  endowed  species,  similarly 
disposed  towards  their  environment,  would  profit  mutually  by  a  geo- 
graphical separation,  which  should  give  to  each  a  range  not  entered  by 
the  other;  and  adjacent  associations  might  thus  be  formed,  alike  in  their 
ecological  make-up  but  different  in  their  species.  It  is  in  some  such 
way  that  we  may  perhaps  explain  a  few  otherwise  unexplained  limita- 
tions of  the  distribution  of  our  Illinois  fishes.  Six  of  our  one  hundred 
and  fifty  Illinois  species  are  so  definitely  limited  to  the  Wabash  drainage 
as  to  suggest  that  there  must  be  some  ecological  barrier  against  their 
spread,  since  there  is  certainly  no  geographical  one,  namely:  brindled 
stonecat,  Schilbcodes  miurus  (Fig.  26  and  Map  LIX)  ;  green-sided  darter, 
Diplesion  blennioides  (Fig.  27  and  Map  LXXXIX)  ;  Notropis  ille'ce- 
brosus  (Fig.  28  and  Map  XXXVII)  ;  silver-mouthed  minnow,  Ericymba 
buccata  (Map  XLVI)  ;  long-eared  sunfish,  Lepomis  megalotis  (Fig.  29 
and  Map  LXXVI) ;  and  Boleichthys  fusiformis  (Fig.  30  and  Map 
XCVIII). 


18 

These  are  all  species  whose  general  distribution  throughout  the 
country  would  lead  us  to  expect  to  find  them  anywhere  in  Illinois. 

Even  more  interesting  is  another  series  of  limitations  upon  the  local 
distribution  of  our  Illinois  fishes,  because  it  seems  to  be  clearly  explain- 
able as  due  to  an  ecological  factor  of  geological  origin — to  the  physical 
character  of  the  surface  soils  of  a  large  part  of  southern  Illinois  cor- 
responding to  the  area  known  as  the  lower  Illinoisan  glaciation.  This 
area  is  notable  for  the  extremely  fine  division  of  its  soil  particles,  due 
to  its  geological  history,  and  for  the  consequent  persistent  and  even 
permanent  muddiness  of  its  waters,  such  that  the  suspended  particles 
cannot  be  completely  separated  by  repeated  filtering  with  the  finest  filter 
paper,  and  do  not  subside  even  after  long  intervals  of  stagnation.  This 
persistent  turbidity  of  the  waters  might  well  be  expected  to  have  an 
effect  to  repel  or  exclude  certain  kinds  of  fishes,  particularly  those  having 
a  special  preference  for  the  clean  water  and  hard  bottom  of  the  lakes  or 
streams  which  they  inhabit.  Other  species,  on  the  other  hand,  which  are 
found  in  muddy  situations  elsewhere,  might  be  expected  to  tolerate  the 
persistently  muddy  waters  of  this  southern  Illinois  district.  An  analysis 
of  our  data  bears  out  this  assumption  in  a  remarkable  way,  a  fact  most 
clearly  shown  by  examples  of  the  distribution  of  species  selected  from 
our  lists  of  those  tolerant,  and  those  intolerant,  of  muddy  waters  gen- 
erally. Compare,  for  example,  our  Illinois  distribution  maps  of  the  stone- 
cat  (Map  LVII),  the  common  sucker  (XVIII),  the  hogsucker  (XIX), 
the  stone-roller  (XXIII),  the  common  shiner  (XLI),  and  the  river  chub 
(LI),  all  rare  or  wanting  in  the  lower  Illinois  glaciation,  with  the  fol- 
lowing six  other  species  freely  distributed  there,  namely :  the  black  bull- 
head (LV),  the  tadpole  cat  (LVIII),  the  chub-sucker  (XVI),  the  blunt- 
nosed  minnow  (XXVIII),  the  golden  shiner  (XXXI),  and  the  long- 
eared  sunfish  (LXXVI)  ;  and  also  the  fact  that  our  statistics  of  ecological 
distribution,  crude  as  they  are,  serve  to  distinguish  these  two  groups 
strongly  with  respect  to  their  relation  to  muddy  situations.  The  fishes  of 
the  first  group,  for  example,  have  occurred  over  muddy  bottom  only  once 
to  nearly  four  times  over  a  bottom  of  mud  and  sand,  while  those  of  the 
second  group  have  occurred  with  about  equal  frequency  in  the  two  situa- 
tions. 

With  these  merely  miscellaneous  illustrations  of  method  and  prod- 
uct, I  must  leave  this  subject,  much  too  large  and  too  complex  for  any 
fairly  comprehensive  treatment,  at  least  by  me,  within  an  hour's  lecture. 
1  am  the  less  disturbed  by  the  fragmentary  character  of  this  discussion 
because  I  know  that  you  have  in  charge  of  your  ecological  studies  a 


19 

leader  in  this  line  of  progress,  abundantly  able  to  make  good  its  defi- 
ciencies, and  especially  competent  to  describe  to  you  the  aims,  methods, 
and  results  of  an  intensive  experimental  study  of  separate  problems  in 
animal  ecology,  quite  in  contrast  to  the  broad  reconnaissance  and  general 
orientation  work  which  naturally  falls  to  the  director  of  a  biological 
survey  of  so  large  an  area  as  the  state  of  Illinois. 


PLATE   I 


ut?j/v 

X 

X 

X 

X 

spuig 

X 

X 

X 

x 

x 

X 

x 

sjuadjag 

x 

x 

x 

X 

S9/j^nj_ 

x 

X 

X 

x 

X 

X 

sdfodpej.  -'sfaojj 

x 

X 

X 

x 

X 

X 

x 

S914SIJ 

X 

X 

X 

X 

x 

X 

x 

X 

X 

x 

x 

x 

X 

sysn/iow 

X 

x 

x 

x 

X 

S-439S-U/ 

X 

x 

X 

X 

x 

x 

x 

x 

sdysijAtej^ 

x 

X 

x 

x 

X 

x 

SUJJO/l/l 

X 

X 

x 

x 

x 

x 

x 

x 

X 

&3&J}SOUJOJU3 

X 

X 

X 

x 

X 

X 

x 

sj9j.lj.oy 

x 

x 

X 

x 

x 

eozojoud 

X 

X 

X 

x 

x 

siueid  JayblH 

x 

X 

X 

9T2&IV 

x 

eudjoeg 

x 

PRINCIPAL 

FOOD  RELATIONS 

OF 

AQUATIC  ORGANISMS 
(ILLINOIS) 

j  Terrestrial  Wastes 

j  Bacteria 

ft 

^ 

Or 

^ 

_£> 

c 
g 

o: 

fc 

•c 

Qr 

^ 

Protozoa 

|  Rotifers 

|  Entomostraca 

e> 

I 

j  Crawfishes 

!  Insects 

|  Mollusks 

Fishes 

CO 

x> 
o 

t 

Turtles 

j  Serpents 

j  Birds 

PLATE  II 


Fisr.  4.    Keel-bellied  Dace  (f7iro,s-on;i<s  erythrogaster) .     X2 


! 


PLATE  III 


Fig.    5.      Upper   jaw   of   young    Gizzard-shad  Fig.    7.  Lower    jaw    of    young    Whitefish    (Core- 

(Uorosoma  cepedianum),   showing  minute  gonus  clupeiformis),  showing  rapta- 

teeth.   X30  torial    teeth.    X30 


Fig.  t>.    Paddle-fish  (Poloydon  spatlmlti) .    Xr/r« 


Fig.  8.     Sheepshead  (Aplodinotus  grunniens).    X 


Fig.  9.    Lake  Sturgeon  (Acipemer  rubicundus) .    XVr,, 


PLATE   IV 


Fig.  10.    Common  Bullhead  (Amfiunix  ?te7iM?oxMs). 


Fig.    11.     Common   Sucker   (Catostomus  commersonii) .    X  !4 


Fig.    12.     Pumpkinseed    (Eupomotis   gibbosus). 


PLATE  V 


Fig,  13.     Lowe 


left  pharrnffeal  jaw  of  Pumpklnseed  (Eupomotte  glbbagus): 

(a),    from    above;    (b),    from    outside 


-maxillary  Teeth  of  Catflshes:     (a).  Noturus  flavus:     (b).  Leptop: 
(c).  Schilheodes  gyrinus:     (d'.  Ameiurus  melas 


PLATE   VI 


Fig.     15.      Sand    Darter    (Aitnuocrypta    pellucidti).     X2 


:.  Hi.    Sucker-mouth 


Fig.    IS.     Blunt-nosed   Minnow    (.Pintrphalrs   notatus). 


PLATE  VII 


\ 


Fig-.  19.    Johnny  Darter  (liiilenxumn  nigrum).     X2 


Fig.  23.    Cattog-a.iter  shumardi    X2 


sr.  21.    Grass  Pickei-Pl  (Exo.r  FcrmicuMuK\     X% 


PLATE   VIII 


Fig.  22.     River  Chub  (Hybopsis  kcntuckiensis). 


Fig.    23.      Bluegill    (Leponiis    pallidus). 


Hlue-siK)tterl  Sunfish  (Lciiomi*  cuanelli, 


PLATE   IX 


Fijr.  25.    Top  Minnow  (Fnndiilu*  dfepar)  male.    X2 


Fig.  2ii.    Brindled  Stonecat  ^ScJiilbeodes  miunt*). 


Fig.  27.    Green-sided  Darter  (Dit>lexi»n  Jile.tniinidex). 


PLATE   X 


Fitr.  :>S.     \ittr»i>i*  iUn-'hi-itxnx.     X1H 


Kig.    29.      Lcng- cared    Sunfish    (Lcpomis    megalotis).     X3A 


Fig.    30.      Bolrichthys   fusiformis    X2 


..Drainage  Canal 
.  County  Sea 


XVIII 

Distribution 

of 

Catostomus 

commersonii 


..111.  and  Mich.  Canal 
..111.  and  Miss.  Canal 
..Drainage  Canal 
.County  Seat 


XIX 

Distribution 

of 

Catostomus 
nigricans 


J1L  and  Mich.  Canal 

.......111.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


XXIII 

Distribution 

of 

Campostoma 
anomalum 


.III.  and  Mich.  Canal 
.III.  and  Miss.  Canal 
.Drainage  Canal 
,  County  Seat 


XXV 

Distribution 


. III  and  Mich.  Canal 

Ill  and  Miss.  Canal 

Drainage  Canal 

.  County  Seat 


XXXI 

Distribution 


I1L  and  Miss.  Canal 

Drainage  Canal 

.  County  Seat 


XXXIV 

Distribution 

of 

Notropis 
heterodon  . 


..  Jll.  and  Mich.  Canal 
....111.  and  Miss.  Canal 
...Drainage  Canal 
.County  Seat 


XXXVII       \Y 
Distribution 

rf 

Notrop 
illecebrosus 


Ul.  and  Mich.  Canal 

III.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


XXXVIII 

Distribution 

of 

Notropis 
hudsonius 


III.  and  Mich.  Canal 

III.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


III.  and  Mich.  On* 

...111.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


XLVI 

Distribution 

of 

Ericymba 
buccata 


..111.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


LI 

Distribution 


LV 

Distribution 

of 

Ameiurus 
melas 


111.  and  Mich.  Canal 

111.  and  Miss.  Canal 

.....Drainage  Canal 
.County  Seat 


..111.  and  Mich.  Canal 
..III.  and  Miss.  Canal 
.Drainage  Canal 
.County  Seat 


III.  and  Mich.  Canal 

III.  and  Miss.  Canal 

Drainage  Canal 

.  County  Seat 


LIX 

Distribution 

of 
Schilbeodes 


III.  and  Mich.  Canal 

III.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


LXXVI 

Distribution 

of 
Lepomis 

megalotis 


. HI.  and  Mich.  Canal 

111.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


LXXXVIII 

Distribution 

of 

Cottogaster 
shumardi 


Jll.  and  Mich.  Canal 

......  III.  and  Miss.  Canal 

Drainage  Canal 

.County  Seat 


LXXXIX 

Distribution 

of 

Diplesion 
blennioides 


111.  and  Mich.  Canal 

III.  and  Hiss.  Canal 

Drainage  Canal 

.County  Seat 


xc 

Distribution 

of 

Boleosoma 
nigrum 


III.  and  Mich.  Canal 

III.  and  Miss.  Canal 

Drainage  Canal 

.  County  Seat 


XCVIII 

Distribution 


Boleichthys 
fusiformis 


...111.  and  Mich.  Canal 
....111.  and  Miss.  Canal 
...Drainage  Canal 
.County  Seat 


